This invention relates to the art of image recording apparatus, particularly an apparatus which, in spite of the fluctuation in the fog density of a recording material to be used, is capable of recording high-quality images that assure satisfactory tone representation over the entire density range.
Image recording apparatus such as copiers and printers are extensively used in diverse fields by employing various recording materials including silver salt photographic materials, thermally processable light-sensitive materials, light- and heat-sensitive recording materials, electrophotographic materials and thermal recording materials. In the case of using a light-sensitive recording material, it is imagewise exposed with recording light such as an optical beam modulated in accordance with the image to be recorded or the light reflected from the original and if a print with a record of a visible (developed) image is necessary, the recording material is subjected to an appropriate development scheme, for example, wet development using various processing solutions if the recording material is a silver salt photographic material, or thermal development if it is a thermally processable light-sensitive material.
In the case of using a thermal recording material, it is imagewise heated to develop color by means of a thermal head having a number of heating elements arranged in one direction or a thermal recording light beam such as a laser beam and this produces a print with a record of a visible image.
In image recording apparatus using the various recording materials described above, a minimum density (highlight) that can be recorded is generally determined by a characteristic of the recording material to be used, namely, its fog density. Consider, for example, a negative recording material that forms color in recorded areas; depending on the characteristics of the recording material, its base density and other factors, there occurs some color formation (density development) in non-recorded areas and this density which is commonly referred to as "fog density" is a minimum density that can be recorded.
The fog density of a virgin recording material (under pre-exposure storage) varies with time, typically toward a higher value. In spite of this fact, the conventional image recording apparatus is operated under constant recording conditions and if the fog density fluctuates, the low-density areas of the recorded image will become "soft" to deteriorate the print quality.